Long HORIZONTAL runs on the main stack?

I'm embarking on a remodel of three bathrooms in a terrible condition, in a 50s house in Southeastern PA.

Bathroom 1: first floor; one shower, one sink and one toilet. No venting anywhere; in fact, it would appear that the sink somehow drains into the shower drain, which in turn drains into the main stack, as there is no separate drain for the sink in evidence. I haven't demolished this yet.

Bathroom 2: second floor; one shower, one sink and one toilet. All fixtures have separate drains into the main stack. None of them are vented.

Bathroom 3: second floor; one tub, one sink and one toilet. All fixtures have separate drains into the main stack. None of them are vented.

The main stack itself discharges horizontally. In fact, the only vertical part of it is the drop from the second floor to the first floor. It is correctly sloped. It is 3" hub-and-spigot cast iron. It's laid out as follows, going downstream:

A) From the vertical roof penetration, it drops between rafters (about 45 degrees) into Bathroom 3. It then turns vertical again for four feet before turning horizontal. This is a sweeping combination of 45 degree elbows.

B) The three fixtures in Bathroom 3 enter the main with wyes: first the toilet, then the tub, then the sink. The main is still horizontal at this point.

C) The three fixtures in Bathroom 2 enter the main with wyes: first the shower, then the sink, then the toilet. The main is still horizontal at this point.

D) After the total horizontal run of about 15 feet, the main drops vertically for about 10 feet into the basement. It again turns horizontal and there is a cleanout at that point.

E) The fixtures in Bathroom 1 enter the main with wyes: first the toilet, then the shower (and presumably sink.) The main is still horizontal at this point, and it continues horizontally for a further 15 feet.

F) The main drops vertically about 18", turns horizontal again (there is a cleanout at this point) and then promptly exits the building underground.

Everything I read and see seems to suggest that the main stack ought to be vertical and only branch drains are allowed to be horizontal. Is that true, and if so, what am I supposed to do with the situation in which I find myself? The issue is not fixture vents, as I'm committed to add those, or any other fixture plumbing details, but the main itself.

Whether the main waste line is vertical or horizontal isn't so much the issue, but where the venting is placed to break up the siphon effect when waster is running by the wyes or tees.
Place a vent between a p-trap and the next connection does that.

You give a lot of information, but most of it is useless as to how to do the replacement. The main can go in any direction it has to, as long as it is going downward, and where the branches connect is immaterial. What makes you think you are qualified to do a job this extensive?

Thank you both for your replies. It's a great relief to learn that nothing is fundamentally wrong with the layout of the main. I'm not qualified to replace the main and was hoping that nothing of the sort would be required (thanks for confirming that) but I didn't want to wind up in the situation where I'm applying bandaids to a gaping wound. I do understand fixture venting pretty well and it's my intent to leave things in a far better shape than I found them, with every fixture being properly vented. Right now, NO fixture is vented, with the possible exception of Bathroom 3 tub and toilet being vented (tub wet-vented) by the main stack. Which brings me to a very specific question: can any portion of the main that carries waste be part of a wet vent? For instance, can the sink in Bathroom 2 wet-vent the shower that joins the main a couple of feet upstream (with the usual constraints with regard to the vertical offset of the drain vis-a-vis the weir of the vented trap)? Can the same sink wet-vent the toilet that joins the main a couple of feet downstream?

Oh boy. I see that my question seems to repeatedly get lost in too much irrelevant detail that I provide, so let me ask it generally without any detail:

Are there any requirements under the IPC for the main governing the use of the main as a horizontal wet vent, beyond those that apply to branch drains?

I am interested in any differences stemming from the fact that this is the main and not a branch. I could not find any reference for special status of the main nor can I think of any reason why there should be - other than the likely higher volume of waste, which is not the case here because we're only talking about "any combination of fixtures within one to two bathrooms on the same level", but I acknowledge that codes have to cover the worst case and can't address every specific situation and that is why I ask.

Here's a quick diagram of the current layout. I apologize for the crudeness of it. Bathroom 3 is the easiest to remedy; I would put the tub and the sink on the same branch, revent the sink, and wet vent the tub via the sink. Bathroom 1 is also relatively easy as I can put a separate vent to atmosphere on that one (it's in a different wing of the house and there's no practical way to tie it to the main vent.) Bathroom 2... is tricky. There's not really ready access to the attic from it from where the sink is located. I have to think on that some.

Now that I have posted the layout, would it be OK to please get some of that advice?

As I mentioned, my intention is to vent each bathroom group separately and fix what else is wrong with the plumbing (for instance, short ells on waste piping plane transitions.) I'll find a way. Once each bathroom group is adequately vented, however, will any issues remain with the main stack, or will it then be adequately vented? (It's 3" all the way from the roof penetration to the street.)

Theoretically each bathroom could use the lav to wet vent the other fixtures in the bath group but the reality may be quite different when you take into account the framing and what is above/below the floor and ceilings. Under the IPC, toilets only require a vent somewhere on the system (toilet to vent distance is unlimited) and the 3" stack through the roof is ok also.

Yes, that lav in Bath 2 is a bit of a stinker when it comes to access to the attic. Worse come to worse, I may put a SMALL soffit along the sink-shower wall to house the sink vent and route it along the ceiling over to where the shower is - so, venting the shower and the sink separately. But there's one thing that I'm looking for under IPC which is the combination drain and vent. I would do this for one fixture only - the lav. In essence:

- vertical drop from lav trap to under floor
- horizontal run to shower drain
- before discharge into the shower drain, a dry vent going up alongside the shower dry vent, and combining 6" above the lav flood level

All of this seems legal according to section 912 of IPC, which says that "up to 8 fixtures" including lavatories (but not showers, hence the separate shower vent) can be drained-vented this way. And one is "up to 8." Thoughts?

Separately from code-correctness, this section would in fact seem to legalize S-traps under specific circumstances, so... even if allowed to do it, should I shy away from this? Alternately, should I additionally stick an AAV on the lav even if it's not required? It seems that this would better protect the trap seal from discharge of the lav itself, while the downstream dry vent would relieve both positive and negative pressure from downstream fixtures and the main.

Without knowing what is there to be dealt with, telling you how to run the vents would never work. Every trap needs to be vented, and the trap to vent distances must fall within the maximum allowed for each pipe size.
Your existing drawing is very nice. If you were to draw in the vents, the pipe sizes, and the distances, then we could tell you whether you are heading in the right direction or not.

I absolutely understand what you're saying and that's why I'm not exactly asking for how to run the vents, but rather: IF there's adequate clearance to maintain code-mandated slopes, distances and fitting arrangements, would a such-and-such approach be allowed and, perhaps more importantly, would it be advisable. Codes are the bare minimum and, for instance, it seems to me that, although the combined drain and vent described under section 912 is ok, there might be issues with it in practice. So, a seasoned pro's "yeah, they'll let you do that but you shouldn't do it anyway because..." or "yep, that'll work well enough" is really what I'm after.

I'll follow up with more specific drawings of what I'm proposing. I'm getting some ideas now so I want to measure things out and make sure I'm not missing something obvious and good due to a blind spot.

My first thought is that you must ask yourself "what is the fault, and what is the cause?". Are you planning to cut open all of your walls to install venting because you are bored, or is there something more to it? Depending on the existing pipe sizes and the amount of build up in the pipes, the combined drain and vent system can certainly have issues, which is likely why it is not currently used in the manner which your house is plumbed.

The basic motivation here is that I'm remodeling all three bathrooms because structurally, code-wise, functionally and aesthetically they are a disaster. The problems with the status quo are a litany and I don't really want to bore anyone with them, but suffice it to say that, by the time I removed the things I had to remove, it was evident that I would have to take it down to bare framing because there just wasn't enough left to try to preserve. This is mainly attributable to a previous "handyman" whose traces I'm doing my best to eradicate. In so doing, I'm hoping to "do things right" and not be another in the line of boneheaded handymen to plague what is essentially a beautifully built house.

The manner in which my house is plumbed doesn't have any venting besides the main stack, so I'm not sure that issues with a combined drain and vent are why it's not currently in use. Although I'm sure that the last remodel (by the "handyman") was in the mid-90s, by which point codes had long required this, none of the fixtures are vented. So sloth and lack of giving a hoot are a more likely explanation for the absence of any particular type of venting, and all venting in general.

The size of the main vent depends on how many fixtures you connect and where you live (snow, cold country often requires a larger section before and through the roof to prevent it from closing due to hoar frost). You can combine vents to minimize roof penetrations if you wish, but it is not required if you can live with multiple roof penetrations. There are specific rules on how and where you can combine vents.

It was quite common to use S-traps, and wet vent things a long time ago, and things generally worked most of the time. There are always exceptions, and remodeling can create some of those situations where they don't work well anymore, so doing it right now when you have the chance is good thinking! (and will make your plumbing inspector appeased, as well!).

Yes, and I'll have to replace some drain plumbing due to inappropriate fittings having been used for plane transitions, too. My hope is that, for the most part, I will be able to actually reduce the number of branches, and one of my top criteria is no new plumbing penetrations through load bearing members. I think I'll be able to accomplish all of this. It will cost a few hundred dollars more in materials and take some more time, but that's not significant compared to the overall scope of the remodel.